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As you probably know, we’ve just spent some time with the fascinating and controversial Volt, and are reporting that during its stay here it returned 126.7 mpg. However, as Editor-in-Chief Angus MacKenzie has pointed out, that number depends spectacularly on how much and how often it’s been charged, as well as the distance of its trips. In EV mode, mpg is infinite, right? Nothing is being burned, right?

Of course, an ‘infinite’ miles-per-gallon number while operating in pure EV mode is a rather silly thing to say. The car is most definitely using energy; it’s a plug-in, after all, not a perpetual-motion machine. The energy being used just happens to be of the electrical sort, in a form usually called ‘kW-hr’. Kilo-Watt-hours? Say Watt?

Let’s back up a bit. A Watt is a unit of power (it’s a little one — 0.134 percent of a horsepower) named for James Watt of steam engine fame. A ‘kilo’ of them means there are a thousand of the little devils, so a kW equals 1.34 horsepower. By multiplying that by ‘hours’, you get a unit of energy, as power x time = energy.

So how much energy is in a gallon of gas? The EPA says a gallon of it equals 33.7 kW-hr — and who are we to argue with them? Consequently, the Volt’s 16-kW-hr battery equals to something like 0.47 gallons of gas. However, as 20 percent of the Volt’s battery capacity is off-limits for longevity reasons, what’s available then is about 12.8 kW-hr, or 0.38 equivalent gallons. So during its 40 or so miles of EV operation, the Volt’s rate of energy use is around 0.32 kW-hr/mile (32.0 kW-hr/100 miles if you don’t like fractions) or 105 miles per equivalent gallon of gas. These energy-equivalent gallons, by the way, are being called ‘egallons’.

At a typical price of $0.12 per kW-hr (quoting the EPA), the Volt’s cost per mile whilst EVing is about 3.8 cents. By comparison, in its gas-burning mode — assuming 40 mpg and a national average of $2.80 per gallon — you’re looking at 7 cents per mile.

As an aside, our complete recharges of the Volt curiously averaged 13.8 kW-hr per plug-in — greater than what the battery presumably allows (12.8 kW-hr). Why? I’d speculate that — unlike pouring gas into a tank — the process of converting electrical energy into a chemical form involves losses (and of course there’s even more of them when it’s converted back again). However, until we get the chance to spend a lot more time with the Volt, let’s regard this as simply anecdotal for now. (For the record, if we use our observed 13.8 kW-hr number instead of 12.8, the Volt’s miles per egallon drops to 98.)

And before any of you start furiously typing that I’m ignoring all those nasty upstream consequences of generating electricity — yes I am. At some point we’ll need to dig into that quicksand, but suffice it to say that the environmental consequence of electrical energy production — specifically, power plant carbon emissions — varies wildly around the country. In areas where electricity is very clean, the Volt is a very good thing. In areas with lots of coal-fired generation, a plug-in’s carbon-badness can easily be worse than a conventional hybrid’s, and maybe no better than a conventional car’s. The logical consequence then is that plug-in hybrids might best be deployed regionally. Ironically, it might not be wise to sell the Volt in the Detroit area where it’s built.

During the car’s stay, we added a total of 58.6 kW-hr (or 1.74 egallons) of electrical energy; meanwhile, 2.36 gallons of the actual liquid stuff was consumed (79.5 kW-hr). Total? 138.1 kW-hr, or 4.1 combined real and e gallons. So while we’re saying that our Volt returned 126.7 mpg over its 299 mile run here, when you account for it electrical consumption, the total number drops to 72.9 miles per combined gallons. Still pretty good – though a normal Prius can get 50 mpg. Use the Volt entirely within its EV zone though, and you just might get that 105 miles per egallon equivalent. No, it’s not infinity, but pretty damned good.

I guess you have to applaud the EPA for embracing such things as equivalent mpg’s (MPGe) and egallons in their proposed new fuel labels. They depict electrical use in familiar-looking forms (you’re still uncomfortable with kW-hrs, aren’t you?) And, bless ’em, the EPA will innovatively offer figures in kW-hrs/100 miles, MPGe, egallons/100 miles, in addition to gas gallons/100 miles and the old-timey (but tragically wrong-way-around) mpg. But gasp — MPGe, egallons, kW-hr/100 miles and mpg — this is getting confusing. It’s certainly inelegant.

But look, energy is energy. And a kW-hr is a unit of energy. A gallon? That’s a unit of volume, for heaven’s sake. Sure, it might make sense to use egallons and MPGe as a kind of transitional terminology. But as more diesel, ethanol, CNG, eventually hydrogen, and certainly lots more electricity itself, stream into our energy supply — why don’t we just cut to the chase and convert all their energy contents to kW-hr and be done with it?

What’s that? You’re wondering what happened to America’s conversion to the metric system?